Activating mechanism for controlling the operation of a downhole tool

ABSTRACT

An activating mechanism for controlling the operation of a downhole tool in a drill string and which is intended to be housed in a portion of the drill string upstream of the downhole tool, in which: the activating mechanism has a first mode in which it allows through-flow of drilling fluid to the downhole tool and a second mode in which through-flow of fluid is blocked; and the activating mechanism has a number of through-flow ports permitting through-flow of drilling fluid in said first mode of the mechanism and which are capable of being blocked by launching a number of flow blocking activator balls down the drill string and which each are of such size and shape that they can block access to said through-flow ports in order to activate the mechanism to the second mode and thereby adjust the downhole tool from one mode of operation to another.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a reissue of U.S. Pat. No. 7,866,397 issued on Jan.11, 2011, application Ser. No. 11/917,621 filed on Jun. 30, 2008 whichis a National Phase Patent Application and claims priority to andbenefit of International Application Number PCT/IB2006/001536, filed onJun. 9, 2006, which claims priority to and benefit of GB ApplicationNumber 0512125.6, filed on Jun. 15, 2005; GB Application No. 0512391.4,filed Jun. 20, 2005; and GB Application No. 0513140.4, the entiredisclosures of which are incorporated herein by reference.

This invention relates to a novel activating mechanism for controllingthe operation of a downhole tool.

The use of a ball-activated mechanism to actuate a downhole tool is wellknown in the exploitation of gaseous and liquid hydrocarbon reserves,and examples include U.S. Pat. Nos. 5,499,687 and 4,889,199, to whichreference is made for a fuller disclosure of this technology.

In this known technology, a large deformable (activating) ball islaunched down the drill string to come into engagement with a ball seatof an axially shiftable sleeve, and which then blocks flow of drillingfluid downwardly through the sleeve (in its through-flow mode) withconsequent increase in pressure upstream of the ball seat. This increasein pressure acts on the ball which then acts downwardly on the sleeve tomove it to a by-pass mode in which fluid flow is diverted laterallythrough one or more by-pass ports in a surrounding main hollow body inwhich the sleeve is mounted.

To deactivate the mechanism, a small hard ball is launched down thedrill string, and which blocks access to the by-pass port and whichresults in a further increase in pressure above the ball seat and whicheventually becomes sufficient to deform the large deformable ball as itis forced downwardly through the ball seat. This then allows the sleeveto return under spring or other biassing back to its normal through-flowmode.

In one aspect the present invention has been developed with a view toprovide a novel activating mechanism (which controls the operation ofthe downhole tool) which can be activated by launching down the drillstring at least one non-deformable activator ball.

In a further aspect the invention provides a novel mechanism utilising acluster of non-deformable activator balls.

According to the invention there is provided a method of controlling theoperation of a downhole tool in a drill string via an activatingmechanism which is housed in a portion of the drill string upstream ofthe downhole tool, in which:

-   (a) the activating mechanism has a first mode in which it allows    through-flow of drilling fluid to the downhole tool and a second    mode in which a through-flow of fluid is blocked; and-   (b) the activating mechanism has a number of through-flow ports    permitting through-flow of drilling fluid in said first mode of the    mechanism and which are capable of being blocked in order to    activate the mechanism to the second mode:

in which a number of flow blocking activator balls are launched down thedrill string and which each are of such a size and shape that they canblock access to said, through-flow ports in order to activate themechanism to the second mode and thereby adjust the downhole tool fromone mode of operation to another.

A method according to the invention therefore enables the operation of adownhole tool readily to be controlled, by launching a number or“cluster” of small hard activator balls from the surface and down thedrill string in order that the mouth of each port can receive arespective ball which thereby blocks flow though the port.

In a first preferred example according to the invention, the activatingmechanism is operative to adjust the downhole tool e.g. an under-reamertool between an operative condition and an inoperative condition. In thecase of an under-reamer tool, the inoperative condition obtains when thereamer blades are in a withdrawn position relative to the body of thetool, and in the operative condition the reamer blades are in a radiallyprojected position relative to the axis of the drill string so as to beengageable with the surrounding formation.

Preferably, the activating mechanism takes the form of a ball-activatedtool, which comprises:

a hollow main body adapted for mounting in a drill string and throughwhich fluid can flow when the tool is an a de-activated mode;

a tubular collet slidably mounted in the main body for movement betweena retained inactive position and a released position correspondingrespectively to the de-activated mode of the tool and an activated mode;

a ball-receiving seat coupled with the collet and arranged to receive anactivating ball launched from the surface and down the drill string toactivate the tool;

spring means arranged in the main body to maintain the collet in theretained position;

a retainer arranged in the main body to engage with and to retain thetubular collet in the inactive position, and to release the collet whenthe tool is activated;

an activating sleeve coupled with the collet for movement therewith toan activating position of engagement with a stop on the main body;

a first by-pass port provided in the collet and communicable internallywith the interior of the collet and externally with the space definedbetween the outer surface of the collet and the inner surface of themain body when the tool is activated; and

a second by-pass port provided in the activating sleeve and communicableexternally with the space defined between the outer surface of thesleeve and the inner surface of the main body, and internally with theinterior of the sleeve, when the sleeve reaches its activating position;

whereby, upon engagement of an activating ball with the seat to activatethe tool, the following sequence takes place:

a. fluid pressure builds-up upstream of the seat;

b. subsequent release of the collet by the retainer;

c. movement of the collet, the ball and the seat, and the activatingsleeve until the sleeve reaches its activating position; and

d. by-pass flow of fluid around the ball and valve seat via the firstand second by-pass ports so that pressurised fluid can flow via the mainbody to activate the related hydraulically operated device.

Conveniently, the activating sleeve is engageable with an internalshoulder provided on the main body to form said stop.

The collet may be coupled with the activating sleeve for movementtherewith via said valve seat.

Conveniently, the ball-activating tool is coupled with a relatedhydraulically operated device, and preferably an under reamer. The mainbody may therefore include a top sub in which the tool is incorporated,and a bottom sub in which the under reamer device is mounted.

The under reamer includes one or more cutter movably mounted in thebottom sub for movement between a withdrawn inoperative position, and anoutwardly projecting operative position.

A flat spring arrangement may be provided to engage via its outer sidewith said cutter, and on its inner side is exposed to fluid flow throughthe main body when the tool is activated, such that the springarrangement can operate to press the cutter outwardly to the operativeposition.

Conveniently, the retainer and the spring means comprise an assembly ofa retainer ring, a set of spacers and spring washers.

The retainer ring is therefore preferably a rigid retaining ring, whichpre-loads the spring washers and also retains the collet. The retainerring holds the collet in place, and fluid dynamics will not affect it.

The ball-activated tool may be activated by launching a single large(non-deformable) ball down the drill string to engage a dedicated seatfor the large ball. Alternatively, the tool may be activated bylaunching a cluster of small hard (non-deformable) balls down the drillstring to engage a seat which is provided with a number of ports eachdedicated to be engaged by a respective one of the small balls.

The first preferred example therefore involves use of a ball-activatedtool which is caused to “pressure-up” the drill string upstream of theseat (which is activated by launch of the large ball, or the cluster ofsmall balls), so that by-pass flow of fluid is conveyed to the downholetool via the activating mechanism and at an increased pressuresufficient to adjust the downhole tool from its inoperative condition toits operative condition.

In a second preferred example according to the invention, aball-activated by-pass tool forms the activating mechanism and which isoperative in a first operating mode to allow through-flow passage offluid to lubricate and cool a drilling bit arranged downstream of theby-pass tool, and in a second operating mode to allow by-pass flow offluid into the surrounding formation, and said tool comprising:

a tubular casing defining a through-flow passage to allow fluid to flowlengthwise of the tool between inlet and outlet ends of the casing andeach being communicable with the drill string;

a transverse by-pass port in the wall of the casing;

a control sleeve mounted in the casing for axial movement between firstand second end positions corresponding to the first and second operatingmodes of the tool;

means biassing the control sleeve towards the first end position so asto block communication with the by-pass port and allow through-flowpassage of fluid in the first operating mode;

a ball-receiving seat provided in the tool to receive a first deformableactivating ball to be launched down the drill-string when it is requiredto adjust the tool from its first operating mode to its second operatingmode, said seat being operative when it receives the activating ball tomove the sleeve from its first end position to its second end positionagainst the action of the biassing means, and in which the firstactivating ball is deformable by the action of a second de-activatingball launched down the drill string so that the first ball can movelengthwise of the tool and thereby allow the sleeve to move back to itsfirst end position under the action of the biassing means when it isrequired to adjust the tool from its second operating mode to its firstoperating mode; and

at least one by-pass seat port provided in the ball-receiving seat andwhich is operative to permit limited flow of fluid through the sleevewhen the latter is in its second end position.

The by-pass seat port therefore allows continued, but limited flow offluid through the sleeve when the latter has been adjusted to its secondend position corresponding to the second operating mode of the tool(by-pass flow of fluid). This enables drilling fluid, usually drillingmud, to continue to flow to the drilling bit, (despite the fact that thetool has been activated to the by-pass mode), and therefore there iscontinued lubrication and cooling of the drill bit so as to prevent, orat least minimise, the risk of permanent damage by over heating of thedrilling bit in high temperature applications.

Conveniently, the control sleeve has a side port which is communicablewith the by-pass port in the wall of the casing when the tool is in itssecond operating mode. Then, upon launching of the second deactivatingball, it comes to rest in a position blocking access to the side port,and thereby interrupts further by-pass flow of fluid. The pressuretherefore increases in the sleeve upstream of the ball-receiving seat,and when a predetermined threshold pressure is exceeded, the firstdeactivating ball is deformed so as to pass downwardly through the seat.It may be arranged also that this action is assisted by downwardpressure of the second deactivating ball on the first activating ball.The sleeve is thus free to return under the action of its biassing meansto the first end position so that the tool takes up again its firstoperating mode.

Preferably, a third type of ball(s) is provided, to be used (in additionto the second deactivating ball) when it is required to revert the toolback to its first operating mode from its second operating mode. Thus,in the second operating mode, the first deformable activating ball isengaged with the ball-receiving seat and main by-pass of fluid isconveyed into the surrounding formation via the by-pass port in the wallof the casing, whereas limited flow of fluid continues to be conveyed tothe drill bit via the by-pass seat port.

The third type of ball is launched from the surface and down the drillstring, and it is of a size such that it can block flow of fluid throughthe by-pass seat port. This enables the pressure upstream of theball-receiving seat to increase still further (in addition to thepressurisation caused by launching of the second deactivating ball), andensures deformation of the first ball and subsequent movement of thecontrol sleeve back to its first operating position.

Conveniently, a set of circumferentially spaced seat ports is provided,and therefore a corresponding cluster of the third type of balls may beprovided to be launched from the surface and to close most, if not allaccess to the seat ports.

Upon deformation of the first activating ball and its movement throughthe seat, this is then followed by the second deactivating ball and theone or more third type of balls.

Conveniently, a ball catcher device may be arranged down stream of theball-receiving seat, to catch at least the first (larger) deformableball and preferably also the second deactivating ball, which ispreferably a hard steel ball. The third type of ball will usually besmaller than the first and second balls, since the seat ports which theyhave to close off will usually be small in diameter, to permit requiredamount of limited continued flow of fluid through the sleeve, andtherefore it will be acceptable for these smaller third type of balls tobe discharged through the drilling bit and into the surroundingformation which is being drilled. Alternatively, the small type of thirdballs may return with the return flow of the drilling mud.

In a third preferred example according to the invention, the activatingmechanism (preferably ball-activated) to actuate a downhole toolcomprises:

a hollow main body adapted for mounting in a drill string and throughwhich flow of fluid to the tool can be routed;

an actuating sleeve defining a through-flow passage and slidably mountedin the main body for movement between positions corresponding to athrough-flow mode and a by-pass mode of the mechanism;

biassing means acting on the sleeve to urge it to its positioncorresponding to the through-flow mode of the mechanism;

a seat providing access to said passage in the through-flow mode of themechanism; and

a deformable activator capable of being launched down the drill stringto engage the seat and thereby cause pressure upstream of the seat toincrease so that the activator moves the sleeve to its positioncorresponding to the by-pass mode of the mechanism;

in which the activator and the seat are arranged to co-operate with eachother, when the activator engages the seat, in such a way thatrestricted flow of fluid through the sleeve is maintained when themechanism is in its by-pass mode.

Therefore, in the by-pass mode of the mechanism, continued thoughrestricted flow of fluid can be maintained to the drilling tool toprevent it from overheating.

Preferably, the hollow body has at least one by-pass port to directfluid flow laterally of the sleeve and the body, when the mechanism isin its by-pass mode. The sleeve is moved by the deformable activator soas to allow access to the by-pass port.

However, to deactivate the mechanism, at least one hard non-deformableactivator, preferably a small hard steel ball, is launched down thedrill string and moves to a blocking position which blocks by-pass flowof fluid to the by-pass port, thereby causing increase in pressureupstream of the seat, but generally not to a level sufficient to movethe deformable activator downwardly through the seat and through thesleeve.

To deactivate the mechanism, a set of small non-deformable activators ispreferably provided, e.g. in the form of small hard balls, and which islaunched down the drill string, and the arrangement of the seat and thedeformable activator (defining limited through-flow passages for fluidwhen they inter-engage) is such that the small activators block thelimited through-flow passages.

The pressure upstream of the seat thus increases further, and eventuallycauses downward movement (accompanied by sufficient inward deformationof the deformable activator) through the seat and the sleeve.

The sleeve then is returned (under its biassing) to its positioncorresponding to the through-flow mode, and the mechanism then revertsto its original mode of operation.

Any suitable downhole tool can have its operation controlled by amechanism according to the invention, and can be actuated andde-actuated by the mechanism in any required way. By way of exampleonly, through-flow of fluid to the tool and via the mechanism canoperate a linearly displaceable mandrel and/or a laterally outwardlymoving actuator, which acts on the tool to control its operation. Returnmovement of the mandrel or the laterally moving actuator can then revertthe tool to its original mode of operation. It should be understood thatone of the modes of the downhole tool may be an inactive mode.

Preferably, the deformable activator comprises a ball-dart combination,in which a ball-like portion at least is deformable and is capable ofseating on the seat, and a dart-like portion can project downwardlythrough the seat. A ball-dart combination can readily be launched down adrill string, and with suitable weighting of the combination, the dartcan pull the ball downwardly, under gravity, and with the darteventually projecting downwardly through the seat and the “ball”engaging the seat.

To provide limited through-flow of fluid in the by-pass mode, it ispreferred that the activator is hollow and is provided with an internalflow control device. This may comprise a number of separate restrictedpassageways, conveniently formed by separate ports in a carrier ring.

Examples of an activating mechanism according to the invention forcontrolling the operation of the downhole tool will now be described indetail with reference to the accompanying drawings, in which:

FIG. 1 is a detailed longitudinal sectional view of a ball-activatedtool for use in a drill string, in order to activate a relatedhydraulically operated device, such as an under-reamer, and showing thetool in a de-activated mode in which fluid flow through the main body ofthe device is permitted.

FIG. 2 is a similar view, but showing the adjustment of the componentsof the tool following launching of an activating ball from the surfacedown the drill string to activate the tool;

FIG. 3 shows, in the separate views, a, b, c, d thereof, fluid flowrelative to the tool in, respectively,

(1) the deactivated mode of the tool,

(2) the launching of an activating ball to initiate activation of thetool,

(3) the build-up of fluid pressure on the activating ball after it hasbeen received by a ball seat and to pressure-up the system, and

(4) adjustment of the components of the tool under the action of thepressure build-up in order to activate the hydraulically operateddevice;

FIG. 4 is a longitudinal sectional view of a top sub and a bottom sub ofa drill string, in which the ball activated tool is mounted in the topsub, and a hydraulically activated downhole tool (e.g. an under-reamer)can be mounted in the bottom sub, and the figure showing the ballactivated tool in its deactivated mode;

FIG. 5, is similar to FIG. 4, but showing the ball activated tool in itsactivated mode, in which it can route pressurised fluid to operate therelated downhole tool (not shown in detail);

FIG. 6 is a longitudinal sectional view, to an enlarged scale, showingthe engagement of a single large deformable ball with the ball seat ofthe axially shiftable sleeve shown in FIGS. 1 to 5;

FIG. 7 is a view, similar to FIG. 6, but showing an alternativearrangement in which the seat incorporates internally a series of smallthrough-flow ports, to be blocked each following launching of a clusterof small non-deformable activator balls down the drill string;

FIG. 8 is a longitudinal view of an alternative example of a deformableactivator which may be launched down the drill string to engage a seatprovided in the axially shiftable sleeve;

FIG. 9 is a view, similar to FIG. 8, showing the internal ports of theactivator of FIG. 8 blocked following launching of the cluster of smallnon-deformable activator balls down the drill string;

FIG. 9a is a view, similar to FIG. 9, showing an alternative arrangementof deformable activator, which is capable of being activated by launchof a large activator ball, and which can subsequently be deactivated bylaunch of two further large balls which block access to by-pass ports,thereby to cause increase in pressure upstream of the deformableactivator, causing the latter to deform and pass downwardly through thevalve seat and deactivate the mechanism;

FIG. 9b is a view, similar to FIG. 9a, showing a similar arrangement;

FIG. 10 is a detailed longitudinal sectional view of a further exampleof a by-pass tool mechanism for use in carrying out a method accordingto the invention;

FIG. 11 is a horizontal cross sectional view of the part of the toolshown in FIG. 10, and showing in more detail by-pass ports provided in aball-receiving seat of a control sleeve of the tool, to permit limitedcontinued flow of fluid to a drilling bit downstream of the tool whenthe tool has been activated to a by-pass mode;

FIG. 12 is a view, similar to FIG. 10, showing three different types ofball for use in activating and deactivating the tool, namely a firstlarge deformable activation ball, a second smaller hard steeldeactivation ball, and a third type of non-deformable (pressure-up)small ball forming a cluster, all for use in a manner to be described inmore detail below; and

FIG. 13 is a view, similar to FIG. 11, showing the third type of ballsclosing-off access to the by-pass seat ports shown in FIG. 11, duringcontrolled adjustment of the tool back to its deactivated mode whennormal through-flow supply of fluid to the drilling tool is resumed.

Referring now to FIGS. 1 to 5 of the drawings, a first example of anactivating mechanism for carrying out a method according to theinvention is designated generally by reference 10 and comprises a hollowmain body 11 (forming a “top sub”) which is adapted for mounting in adrill string in order to activate a related hydraulically operateddevice (shown in more detail in FIGS. 4 and 5). The device shown inFIGS. 4 and 5 is an under reamer, but it should be understood that thisis merely one example of a related hydraulically operated device whichcan be activated by the ball-activated tool of the invention.

The hollow main body 11 permits through flow of fluid to take place whenthe tool is in a de-activated mode, as shown in FIG. 1. A tubular collet12 is slidably mounted in the main body 11 for movement between aretained inactive position (as shown in FIG. 1), and a released position(as shown in FIG. 2) corresponding respectively to the de-activated modeof the tool and the activated mode.

A ball-receiving seat 13 is coupled with the collet 12 and is arrangedto receive an activating ball launched from the surface and down thedrill string to activate the tool. A ball 14 is shown in FIG. 2 inengagement with the seat 13, and with the tool components adjusted to areleased active position, which causes activation of the tool.

Spring means 15 in the form of a set of spring washers is arranged inthe main body 11 and which act to maintain the collet 12 in the retainedposition shown in FIG. 1. The spring means 15 cooperate with a retainer,in the form of a rigid retainer ring 16, and two end spacers 17, inorder to retain the tubular collet 12 in the inactive position. However,upon activation of the tool, as will be described in more detail below,the collet 12 is released by the retainer ring 16, and against theopposition of the spring means 15, in order that the collet 12 can moveto a released position which initiates adjustment of the tool to theactivated mode.

An activating sleeve 18 is coupled with the collet 12 for movementtherewith to an activating position of engagement with a stop providedon the main body, as shown in FIG. 2. In the embodiment of FIG. 2, thestop is provided by an internal shoulder 19 which limits the movement ofthe sleeve 18 and collet 12 to the active position shown in FIG. 2.

Although not shown in detail in FIGS. 1 and 2, a first by-pass port isprovided in the collet 12, and which communicates internally with theinterior of the collet and externally with the space defined between theouter surface of the collet 12 and the inner surface of the main body 11when the tool is activated. This will be described in more detail belowwith reference to FIG. 3.

There is also a second by-pass port provided in the activating sleeve 18(also not shown in FIGS. 1 and 2), and which communicates externallywith the space defined between the outer surface of the sleeve 18 andthe inner surface of the main body 11, and internally with the interiorof the sleeve, when the sleeve reaches its activating position shown inFIG. 2.

Referring now to FIG. 3, the four views a, b, c and d show successivestages of adjustment of the tool between the deactivated mode and thefully activated mode of the tool.

In FIG. 3a, normal fluid flow down the drill string and through theinterior of the main body 11 is permitted, and during this time therelated hydraulically operated device (the under reamer) remainsinoperative.

FIG. 3b shows initiation of adjustment of the tool to its activatedmode, which is caused by launching activating ball 14 from the surfaceand down the drill string, to engage seat 13.

FIG. 3c shows the components of the tool still in the deactivatedpositions, but with the ball 14 engaged with the seat 13, pressurebuilds-up upstream of the ball and pressures up the system until suchtime as the fluid pressure force acting on the ball 14 causes the collet12 to be released by the retainer ring 16, so that the assembly ofcomponents 12, 13 and 18 move as a unit to the position shown in FIG.3d, such position being defined by inter-engagement between the outerend 20 of activating sleeve 18 with shoulder 19.

As shown in FIG. 3c, the potential flow of fluid through the system,shown by arrow 29, is initially prevented by virtue of the seating ofball 14 on the seat 13, until such time as the pressure build-up issufficient to cause the collet 12 to be released by the retaining ring16. FIG. 3d then shows the fluid flow path through the system, which isat a higher pressure than the through flow in the deactivated mode ofFIG. 3a, and such pressure is sufficient to trigger operation of theunder reamer.

As can be seen in FIG. 3d, the fluid flow effectively by-passes the ball14 engaged with seat 13, by first passing outwardly from the interior ofcollet 12 through one or more first by-pass ports 21 to the space 22between the outer surface of collet 12 and the inner surface of mainbody 11. The by-pass flow then returns to the interior of the main body11 via one or more second by-pass ports 23 in the activating sleeve 18.This resumed through-flow of fluid, at enhanced pressure, and shown byarrow 24, then passes to a hydraulically operated downhole tool(preferably an under-reamer), arranged below the tool 10, to initiateoperation of the latter.

FIG. 4 illustrates schematically a top sub 25 in which the tool 10 ismounted, and a bottom sub 26 in which a hydraulically operated tool 27,such as an under reamer can be mounted. FIG. 4 shows the tool in itsde-activated position, and FIG. 5 is a similar view to FIG. 4, butshowing the tool 10 in its activated position in which it can routepressurised fluid to operate the tool 27 e.g. an under reamer.

FIG. 6 is a longitudinal sectional view, to an enlarged scale, showingthe engagement of a large non-deformable activation ball 14 with aninternal ball receiving seat 13 of the axially shiftable sleeve 12 whichis described above and shown in more detail in FIGS. 1 to 5.

FIG. 7 shows an alternative arrangement of ball-receiving seat 13a,which is provided with an internal flow control device comprising a setof small through-flow ports, each of which is capable of having accessto it blocked following launch of a cluster of small non-deformableactivator balls 14a down the drill string.

FIGS. 8 and 9 show another example of an activator system foractivating, and deactivating, a mechanism which controls the operationof the downhole tool. By way of example only, it will be assumed thatthe activator system shown in FIGS. 8 and 9 is being used in relation toactivation of the mechanism and downhole tool described above withreference to FIGS. 1 to 5.

FIGS. 8 and 9 are longitudinal sectional views of a deformable activatorin the form of ball-dart combination, which takes the place of the largenon-deformable ball 14 described above. There is therefore shown inFIGS. 8 and 9 a deformable activator which is designated generally byreference 50 having a ball-like portion 51 which engages the seat 13,and a dart-like portion 52 projecting downwardly therefrom. Theball-like portion 51 engages the seat 13, and the dart-like projection52 projects downwardly therefrom and through the seat. The activator 50is hollow, defining a limited or restricted through-flow passage, sothat when the activator engages the seat, it causes pressure upstream ofthe seat to increase so that the activator moves the sleeve 12downwardly to a position corresponding to the by-pass mode of themechanism.

However, the activator 50 and the seat 13 are arranged to cooperate witheach other, when the activator 50 engages the seat, in such a way thatrestrictive flow of fluid through the sleeve 12 is maintained when themechanism is in its by-pass mode.

Therefore, in the by-pass mode of the mechanism, continued thoughrestricted flow of fluid can be maintained to the drilling tool tolubricate and prevent it from overheating.

The activator 50 incorporates a flow control device 53 arrangedinternally thereof, and comprising a ring formed with a number of portsforming separate restricted passageways.

FIG. 8 shows the activator 50 before employment of any activating andde-activating devices. When engaged with the seat 13, limitedthrough-flow of fluid is allowed, even though the mechanism is in theby-pass mode. However, to commence the deactivation of the mechanism, atleast one hard non-deformable activator is used, preferably a small hardsteel ball, and which is launched down the drill string and moves to ablocking position which blocks by-pass flow to the by-pass port. Thiscauses increase in pressure upstream of the seat 13, but generally notto a level sufficient to move the deformable activator 50 downwardlythrough the seat 13 and through the sleeve 12.

To complete the deactivation of the mechanism, a set of smallnon-deformable (pressure-up) actuators is provided, e.g. in the form ofsmall hard balls 54 launched down the drill string. The arrangement ofthe seat 13 and the deformable actuator 50 is such that the balls 54block the limited through-flow passages. The pressure upstream of theseat 13 therefore increases further, and eventually causes downwardmovement (accompanied by sufficient inward deformation of actuator 50)through the seat 13 and the sleeve 12.

The sleeve 12 is then returned to its position corresponding to thethrough-flow mode, and the mechanism then reverts to its original modeof operation. In addition to the provision of restricted passages in theflow control device 53, outlet ports 55 are provided in a nose portionof the dart-like portion 52.

The activator 50 therefore incorporates a ball-port ring within thedart-like portion, which allows a split flow situation for the drillingfluid used, in that a main part of the fluid passes via the by-pass portupon activation, whereas limited flow can be maintained via the flowcontrol device in the activator. When it is necessary to close the tool,deactivation ball or balls are dropped down the drill string, followedby a cluster of non-deformable pressure-up balls. Two largerdeactivation balls will plug up the main bypass port, whereas thesmaller non-deformable (pressure-up) balls will come down into the portsin the dart-like portion, allowing the pressure above the dart to buildup. Pressure will increase above the small balls until such time as theplastics material from which at least the ball-like portion of theactivator 50 is formed can deform and allow the entire activator to blowdownwardly through the seat and the sleeve, and be caught within asuitable “ball catcher” device (now shown) arranged downstream thereof.The balls fall through on top of the dart, and this operation can berepeated when required.

The dart may also be adapted to utilise a flap of valve or retentionmechanism, to retain the small balls within the dart.

Referring now to FIGS. 9a and 9b, this shows further embodiments ofdeformable activators 50a and 50b respectively, and which are generallysimilar to the deformable activator 50 described above with reference toFIGS. 8 and 9. Corresponding parts are given the same referencenumerals.

The deformable activators 50a and 50b can be launched down a drillstring to engage the valve seat, and launch of a large activator ball115 can block downward flow of fluid through the activator, and therebypressure upstream of the activator increases thereby shifting themechanism axially to an alternative mode of operation, whereby throughflow of fluid is blocked. Alternatively, small bleed passages may beprovided, to allow limited through flow of fluid to cool and lubricatethe drilling bit arranged downstream thereof. However, the main portionof the drilling fluid can then pass transversely through outlet ports112.

When it is required to revert the activating mechanism to thedeactivated mode, e.g. for normal operation of the downhole tool,further deactivating ball(s) 117 is launched down the drill string, toblock access to the respective outlet port(s) 112. This then causes thepressure upstream of the activator 50a, 50b to increase, and thedeformable portion 51 of the activator then yields under this load,thereby allowing the entire activator to pass downwardly through thevalve seat, and allow the mechanism to revert to its deactivated mode.

The deformable activators 50, 50a, 50b disclosed herein effectively area form of deformable dart, and having an external resilient ring, whichmay be made of the same material as the plastics material from whichdeformable activator balls are usually made, so that the deformable ringcan shear under load, to allow the dart to pass downwardly through thevalve seat.

The ring therefore forms a seal on the outer circumference of the dart,and is assembled this way so as to allow for a large area of bypassthrough the tool when the latter is in the activated mode. This allows alarge volume to be pumped downwardly to the operating drill bit, as wellas still a large volume laterally through the nozzles in the sideport(s) of the tool.

Referring now to FIGS. 10 to 13, a still further example of ballactivated mechanism will now be described, for use in carrying out amethod according to the invention. There is show in detail only part ofa ball activated by-pass tool, designated generally by reference 110,and comprises an outer tubular casing 111 provided with at least oneby-pass port 112 in its side wall, and an axially shiftable controlsleeve 113 provided with a ball-receiving seat 114.

The by-pass tool 110 is insertable into a drill string, and is operativein a first operating mode to allow through flow passage of fluid tolubricate and cool a drilling bit provided downstream of the by-passtool, and in a second operating mode to allow by-pass flow of fluid intothe surrounding formation. The general construction and operation of theby-pass tool 110 may be as disclosed in more detail in U.S. Pat. No.5,499,687, and WO01/90529, the disclosure of which is incorporatedherein by this reference.

The tubular casing 111 defines a through flow passage to allow drillingfluid, usually drilling mud, to flow lengthwise of the tool 110 betweeninlet and outlet ends of the casing, and each being communicable withthe drill string. The control sleeve 113 is mounted in the casing 111for axial movement between first and second end positions correspondingto the first and second operating modes of the tool. FIG. 10 shows thetool in its second operating mode, permitting by-pass flow of fluid,following activation of the tool by launching of a first largeactivating ball, as described in more detail below.

Biassing means is provided (not shown), preferably in the form of acompression spring, which biasses the control sleeve 113 towards thefirst end position so as to block communication with the by-pass port112 and allow through flow passage of fluid in the first operating mode.

The ball-receiving seat 114 provided in the tool 110 can receive a firstdeformable activating ball 115, launched from the surface and down thedrill string, when it is required to adjust the tool from its firstoperating mode to its second operating mode. The seat 114 is operativewhen it receives the activating ball 115, as shown in FIG. 10, to movethe sleeve 113 from the first end position to the second end positionand against the action of the biassing means.

In the second end position of the control sleeve 113, a side port 116 inthe wall of the sleeve 113 communicates with the by-pass port 112, toallow by-pass flow of fluid when required. Only a single by-pass port112 and side port 116 are shown, but evidently more than one port may beprovided, and other means of communication may be used.

When it is required to deactivate the tool, a second deactivating ball117 (see FIG. 12) is launched down the drill string, with the resultthat the first ball 115 can move lengthwise of the tool 110, preferablyto be received by a ball catcher device (not shown), and thereby allowthe sleeve to move back to its first end position under the action ofthe biassing means. Thus, the second deactivating ball 117 blockscommunication to the side port 116, and therefore interrupts by-passflow via the by-pass port 112, and therefore the pressure upstream ofthe seat 114 increases, and when a threshold pressure is exceeded, thelarge deformable ball 115 deforms under the pressure load so as to movedownwardly through the ball seat 114.

The description thus far generally corresponds to that which isdisclosed in more detail in U.S. Pat. No. 5,499,687, and WO01/09529.However, there now follows detailed description of a simple, but highlyeffective additional feature, forming a preferred embodiment of theinvention.

Thus, to provide limited, but continued flow of fluid through the sleeve113, when the latter has been adjusted to its second end positioncorresponding to the second operating mode of the tool (by-pass flow offluid), an additional by-pass arrangement is provided. This enablesdrilling fluid, usually drilling mud, to continue to flow to thedrilling bit, and thereby continue lubrication and cooling of thedrilling bit and prevent, or at least minimise, the risk of permanentdamage by overheating to the drilling bit in high temperatureapplications.

The additional by-pass arrangement takes the form of at least oneby-pass seat port 118 provided in the ball-receiving seat 114. In theillustrated arrangement, a circumferentially spaced set of arcuate slotsare formed in the seat 114, to form the means providing continued, butlimited flow of by-pass fluid, when the tool is operating in its by-passmode shown in FIG. 10.

The subsequent launching of the deactivating ball 117 will stillinterrupt main by-pass flow via by-pass port 112, and subsequentincrease in pressure upstream of the large deformable ball 115, andwhich can increase to a sufficient extent to allow deformation of theball 115 and downward movement through the seat 114, to deactivate thetool, despite the fact that some of the fluid will be flowing downwardlythrough the tool, in limited manner, via the by-pass seat ports 118.

However, to ensure that the pressure being bled-off via the by-pass seatport 118 does not prevent deactivation of the tool by launching of thesecond deactivating ball 117, it is preferred to provide a third type ofball for use with the tool 110. This third type of ball is to be used,in addition to the second deactivating ball 117, when it is required torevert the tool 110 back to its first operating mode from its secondoperating mode. The third type of ball is a small non-deformable ball,and preferably supplied in a cluster of balls 119, as shown in FIGS. 12and 13.

The balls 119 are of such a size that, when used together in a cluster,they can block flow of fluid through the by-pass ports 118, andtherefore enable the pressure upstream of the seat 114 to increase stillfurther (in addition to the pressurisation caused by launching of thesecond deactivating ball 117), and thereby ensure the deformation of thefirst activation ball 115 and subsequent downward movement through theseat 118 and followed by upward movement of the control sleeve 113 underthe action of its spring biassing back to the first operating position.

In the illustrated embodiment, more than one of the balls 119 is used inorder to close off each of the by-pass seat ports 118. However, otherarrangements are possible, including single balls 119 each closing off arespective by-pass port, although this is not shown in the illustratedembodiment.

Upon deformation of the first activating ball 115 and its movementthrough the seat 114, this is then followed by the second deactivatingball 117 and the third type of balls 119.

Conveniently, a ball catcher device (not shown) is arranged downstreamof the scat 114, to catch at least the first (larger) ball 115, andpreferably also the second deactivating ball 117, which is a hard steelball. The third type of ball 119 is smaller in diameter than the otherballs, in view of the size and shape of the by-pass ports 118, andtherefore it will be acceptable for the balls 119 to be dischargedthrough the drilling bit and into the surrounding formation beingdrilled, or to be returned to surface with the return flow of drillingmud.

The by-pass tool 110 as described above therefore enables the tool to beactivated by dropping the first activation ball 115, to initiate mainby-pass flow via the by-pass port 112 in the casing, while stillallowing a limited flow of fluid to pass around the activation ball 115and through the seat ports 118. There is therefore a split flowsituation, in which the main by-pass flow is conveyed via the by-passport 112, while a smaller proportion of the fluid passes downwardlythrough the valve seat 114 via the seat ports 118.

However the provision of the seat ports 118 does mean that some of thepressure above the seat is bled-off, and therefore this reduces thepressure available to deform the activation ball 115, after launch ofthe de-activating ball 117. It is for this reason that the third type ofballs 119 are provided, which are able to blank-off at least the majorpart of the access to the seat ports 118.

The typical sequence of operations therefore would be as follows: dropthe plastics activation ball 115, to open up the tool, and pump mainby-pass fluid for as long as the operator requires, but with split flowand some of the flow going down through the sleeve to lubricate and coolthe drilling bit, in addition to the main by-pass flow via thecirculating ports above the ball 115.

To deactivate the tool, the steel deactivation ball 117 is dropped downthe drill string, in the case of a single ported tool, or twodeactivation balls are dropped in the case of a dual ported tool.Thereafter, the non-deformable pressure-up balls (the third balls 119)are dropped down the drill string. When the steel deactivation ball(s)117 closes access to the side port 16 above the ball 115, the systemstarts to pressure-up, and further flow now only continues around thedeformable activation ball 115, and via the seat ports 118. When thenon-deformable pressure-up third balls 119 reach the seat 114, theyplug-up the seat ports 118, allowing the operator, or allowing thesystem, to pressure-up to a greater extent and thereby ensuredeformation of the main activation ball 115. Ball 115 then passesdownwardly, upon deformation, through the seat 114, and this is followedby the spring biassing urging the control sleeve 113 to its first endposition (sleeve closed position), and the steel deactivation ball(s)117 can then fall downwardly through the ball seat 114, following thedeformable activation ball 115, and both of these can be caught by aball catcher device (not shown). However, the smaller, non-deformableand pressure-up balls 119 are sufficiently small that they can bedisplaced downwardly through the drilling tool and through the drillingbit, and out into the surrounding formation. Alternatively, balls 119can return to surface via the return flow of drilling mud.

The invention claimed is:
 1. An activating mechanism for controlling theoperation of a downhole underreamer tool in a drill string and which isintended to be housed in a portion of the drill string upstream of thedownhole underreamer tool, in which: (a) the activating mechanism has afirst mode in which it allows through-flow of drilling fluid to thedownhole underreamer tool in an operative condition and a second mode inwhich through-flow of fluid is blocked in an inoperative condition; (b)the activating mechanism has a number of through-flow ports permittingthrough-flow of drilling fluid in said first mode of the mechanism andwhich are capable of being blocked by launching a number of flowblocking activator balls down the drill string and which each are ofsuch size and shape that they can block access to said through-flowports in order to activate the mechanism to the second mode and therebyadjust the downhole underreamer tool from one mode of operation toanother; and (c) the inoperative condition is obtained when the reamerblades are in a withdrawn position relative to the body of the tool, andin the operative condition the reamer blades are in a radially projectedposition relative to the axis of the drill string so as to be engageablewith the surrounding formation.
 2. A mechanism taking the form of aball-activated tool for use in a drill string in order to activate arelated hydraulically operated device and which comprises: a hollow mainbody adapted for mounting in a drill string and through which fluid canflow when the tool is a in a de-activated mode; a tubular colletslidably mounted in the main body for movement between a retainedinactive position and a released position corresponding respectively tothe deactivated mode of the tool and an activated mode; a ball-receivingseat coupled with the collet and arranged to receive an activating balllaunched from the surface and down the drill string to activate thetool; spring means arranged in the main body to maintain the collet inthe retained position; a retainer arranged in the main body to engagewith and to retain the tubular collet in the inactive position, and torelease the collet when the tool is activated; an activating sleevecoupled with the collet for movement therewith to an activating positionof engagement with a stop on the main body; a first by-pass portprovided in die collet and communicable internally with the interior ofthe collet and externally with the space defined between the outersurface of the collet and the inner surface of the main body when thetool is activated; and a second by-pass port provided in the activatingsleeve and communicable externally with the space defined between theouter surface of the sleeve and the inner surface of the main body, andinternally with the interior of the sleeve, when the sleeve reaches itsactivating position; whereby, upon engagement of the activating ballwith the seat to activate the tool, the following sequence takes place:a. fluid pressure builds-up upstream of the seat; b. subsequent releaseof the collet by the retainer; c. movement of the collet, the ball andthe seat, and the activating sleeve until the sleeve reaches itsactivating position of engagement with stop; and d. by-pass flow offluid around the ball and valve seat via the first and second by-passports so that pressurised fluid can flow via the main body to activatethe related hydraulically operated device.
 3. A mechanism according toclaim 2, in which the activating sleeve is engageable with a shoulderprovided internally of the main body to form said stop.
 4. A mechanismaccording to claim 2, in which the collet is coupled with the activatingsleeve for movement therewith via said valve seat.
 5. A mechanismaccording to claim 2, in which the main body includes a top sub in whichthe tool is incorporated, and a bottom sub in which a tool, is mounted.6. A mechanism according to claim 2, in which the ball-activated tool iscapable of being activated by launching a single large non-deformableball down the drill string to engage the seat dedicated to the largeball.
 7. A mechanism according to claim 2, in which the ball activatedtool is capable of being activated by launching a cluster of small hardnon deformable balls down the drill string to engage a seat which isprovided with a number of ports each dedicated to be engaged by arespective one of the small balls.
 8. A ball-activated by-pass tool foruse in a drilling operation, and which is insertable into a drill stringand which is operative in a first operating mode to allow through-flowpassage of fluid to lubricate and cool a drilling bit arrangeddownstream of the by-pass tool, and in a second operating mode to allowby-pass flow of fluid, and said tool comprising: a tubular casingdefining a through-flow passage to allow fluid to flow lengthwise of thetool between inlet and outlet ends of the casing and each beingcommunicable with the drill string; a transverse by-pass port in thewall of the casing; a control sleeve mounted in the casing for axialmovement between first and second end positions corresponding to thefirst and second operating modes of the tool; means biasing the controlsleeve towards the first end position so as to block communication withthe by-pass port and allow through-flow passage of fluid in the firstoperating mode; a ball-receiving seat provided in the tool to receive afirst deformable activating ball to be launched down the drill-stringwhen it is required to adjust the tool from its first operating mode toits second operating mode, said seat being operative when it receivesthe activating ball to move the sleeve from its first end position toits second end position against the action of the biasing means, and inwhich the first activating ball is deformable by the action of a secondde-activating ball launched down the drill string so that the first ballcan move lengthwise of the tool and thereby allow the sleeve to moveback to its first end position under the action of the biasing meanswhen it is required to adjust the tool from its second operating mode toits first operating mode; and at least one by-pass seat port provided inthe ball-receiving seal and which is operative to permit limited flow offluid through the sleeve when the latter is in its second end position,wherein the control sleeve has a side port which is communicable withthe by-pass port in the wall of the casing, when the tool is in itssecond operating mode.
 9. A tool according to claim 8, in which thecontrol sleeve has a side port which is communicable with the by-passport in the wall of the casing, when the tool is in its second operatingmode.
 10. A tool according to claim 8, including a third type of ballfor use, in addition to the second de-activating ball when it isrequired to revert the tool back to its first operating mode from itssecond operating mode.
 11. A tool according to claim 10, in which a setof circumferentially spaced slots are formed in the ball seat, to formsaid seat port, and a cluster of said third type of balls is provided tobe launched from the surface, to close-off at least a major part ofaccess to said slots.
 12. A tool according to claim 8, in which a ballcatcher device is arranged downstream of the ball receiving seat, tocatch at least said first activation ball and optionally said secondde-activating ball.
 13. An activating mechanism for controlling theoperation of a downhole tool and which comprises: a hollow main bodyadapted for mounting in a drill-string and through which fluid to thetool can be routed; an actuating sleeve defining a through-flow passageand slidably mounted in the main body for movement between positionscorresponding to a through-flow mode and a by-pass mode of themechanism; biasing means acting on the sleeve to urge it to its positioncorresponding to the through-flow mode of the mechanism; a seatproviding access to said passage in the through-flow mode of themechanism; and a deformable activator capable of being launched down thedrill-string to engage the seat and thereby cause pressure upstream ofthe seat to increase so that the deformable activator moves the sleeveto its position corresponding to the by-pass mode of the mechanism; inwhich the deformable activator and the seat are arranged to co-operatewith each other, when the deformable activator engages the seat, in sucha way that restricted flow of fluid through the sleeve is maintainedwhen the mechanism is in its by-pass mode; the deformable activatorbeing a deformable dart comprising: a dart body having a outer sidesurface portion with a circumferential groove in the side surfaceportion; an external deformable ring which is inwardly deformable, theexternal deformable ring being mounted in the circumferential groove andprotruding outwardly therefrom such that the deformable ring is capableof seating on said seat with at least a portion of the dart body belowthe circumferential groove projecting downwardly through the seat,wherein the external deformable ring is capable of deforming under load,thereby allowing the entire deformable activator to pass downwardlythrough the seat; wherein the deformable ring has an outer surface, atleast a portion of which tapers downward and radially inwardly.
 14. Amechanism according to claim 13, in which the hollow body has at leastone by-pass port to direct fluid flow laterally of the sleeve and thebody when the mechanism is in its by-pass mode, and with the sleevebeing moved by the deformable actuator so as to allow access to theby-pass port.
 15. A mechanism according to claim 13, in which anon-deformable activator is provided, which is capable of being launcheddown the drill string and to move to a blocking position which blocksby-pass flow of fluid, and thereby causes increase in pressure upstreamof the seat.
 16. A mechanism according to claims 13 claim 19, includinga set of small non-deformable pressure-up activators, capable of beinglaunched down the drill string, and the arrangement of the seat and thedeformable activator is such that the each pressure-up activators arethen activator is capable of blocking the limited through-flow passagesone of the ports.
 17. A mechanism according to claims 13, in which thedeformable activator comprises a ball-dart combination, in which aball-like portion at least is deformable and is capable of seating onsaid seat, and a dart-like portion is capable of projecting downwardlythrough the seat.
 18. A mechanism according to claim 17, in which thedeformable activator is hollow and is provided with an internal flowcontrol device.
 19. A mechanism according to claim 18 39, in which theflow control device comprises a carrier ring provided with a number ofseparate ports.
 20. A mechanism according to claim 13 36, in which: thedeformable activator is capable of being launched down the drill stringto engage the seat, so that pressure upstream of the seat increases andthe deformable activator moves the sleeve so that the mechanism takes upits by-pass mode, while allowing limited flow of fluid to the tool to bemaintained in one of its modes of operation, and said mechanism alsoincluding: a small hard ball(s) capable of being launched down the drillstring when it is required to provide by-pass flow of fluid; and one ormore de-activator ball(s) capable of being launched down the drillstring to block limited flow of fluid through the sleeve, when it isrequired to revert the tool to another of its modes, such that thepressure upstream of the seat increases and causes downwarddisplacement, accompanied by inward deformation, of the deformableactivator through the seat so that the mechanism reverts to itsthrough-flow mode.
 21. An activating mechanism for controlling theoperation of a downhole tool and which comprises: a hollow main bodyadapted for mounting in a drill string and through which fluid to thetool can be routed; an actuating sleeve defining a through-flow passageand slidably mounted in the main body for movement between positionscorresponding to a through-flow mode and a by-pass mode with amechanism; biasing means acting on the sleeve to urge it to its positioncorresponding to the through-flow mode of the mechanism; a seatproviding access to said passage in the through-flow mode of themechanism; a deformable activator capable of being launched down thedrill string to engage the seat and thereby cause pressure upstream ofthe seat to increase; an activator ball capable of being launched downthe drill string to engage a through-flow seat at an upstream end of thedeformable activator, thereby to increase, or still further increase thepressure upstream of the deformable activator and thereby cause thesleeve to move to its position corresponding to the by-pass mode of themechanism; and at least one deactivating ball capable of being launcheddown the drill string to block access to an outlet port thereby toincrease the pressure upstream of the deformable activator until suchtime as the deformable activator deforms itself to pass the entiredeformable activator downwardly through the seat to allow the mechanismto revert to its through-flow mode, the deformable activator being adeformable dart comprising: a dart body having an outer side surfaceportion with a circumferential groove in the side surface portion; anexternal deformable ring which is inwardly deformable, the externaldeformable ring being mounted in the circumferential groove andprotruding outwardly therefrom such that the deformable ring is capableof seating on said seat with at least a portion of the dart body belowthe circumferential groove projecting downwardly through the seat;wherein the deformable ring has an outer surface, at least a portion ofwhich tapers downward and radially inwardly.
 22. An activating mechanismfor controlling the operation of a downhole tool and which comprises: ahollow main body adapted for mounting in a drill-string and throughwhich fluid to the tool can be routed; an actuating sleeve defining athrough-flow passage and slidably mounted in the main body for movementbetween positions corresponding to a through-flow mode and a by-passmode of the mechanism; biasing means acting on the sleeve to urge it toits position corresponding to the through-flow mode of the mechanism; aseat providing access to said passage in the through-flow mode of themechanism; and a deformable dart capable of being launched down thedrill-string to engage the seat and thereby cause pressure upstream ofthe seat to increase so that the deformable dart moves the sleeve to itsposition corresponding to the by-pass mode of the mechanism; in whichthe deformable dart and the seat are arranged to co-operate with eachother, when the deformable dart engages the seat, in such a way thatrestricted flow of fluid through the sleeve is maintained when themechanism is in its by-pass mode; and in which the deformable dartcomprises a dart body having a top end, a cylindrical outer sidesurface, a bottom end and a circumferential groove having an upperannular surface and a lower annular surface; a deformable ring capableof inward deformation mounted in the circumferential groove andprotruding outwardly therefrom such that the deformable ring is capableof seating on said seat, with a portion of the dart body being capableof projecting downwardly through the seat, and when the deformable dartis seated on the seat and there is sufficient pressure upstream of thedeformable dart, the deformable ring is capable of deforming to allowthe entire deformable dart to pass downwardly through the seat, whereinthe deformable ring has an outer surface, at least a portion of whichtapers downward and radially inwardly.
 23. A mechanism according toclaim 22 in which the dart body is solid.
 24. A mechanism according toclaim 23 wherein the bottom end of the dart body has an outer surfacethat tapers downward and radially inwardly.
 25. A mechanism according toclaim 24 wherein the bottom end of the dart body has a rounded outersurface.
 26. A mechanism according to claim 22 in which the cylindricalouter side surface is defined by a tubular side wall, and the dart bodycomprises a flow through passageway extending from an inlet opening atthe top end of the dart body to an outlet opening at the bottom end ofthe dart body.
 27. A mechanism according to claim 26 in which thediameter of the flow through passageway is greater than half of thediameter of the dart body.
 28. A mechanism according to claim 26 inwhich the flow through passageway comprises an upper portion having afirst diameter and a lower portion having a second diameter about thesame as the first diameter.
 29. A mechanism according to claim 26 inwhich the flow through passageway has an upper portion and a lowerportion in which the lower portion divides into multiple separate outletpassageways, each having a diameter less than the diameter of the upperpassageway.
 30. A mechanism according to claim 22, wherein the dart bodycomprises at least two components including an upper component and alower component which are assembled together into a unitaryconstruction.
 31. A mechanism according to claim 30, wherein the lowercomponent comprises a tubular, generally cylindrical outer side wall andthe upper component comprises a tubular, generally cylindrical innerside wall sized to fit closely within the tubular, cylindrical outerside wall of the lower component and when assembled, at least a portionof the inner sidewall of the upper component is located within the outersidewall of the lower component.
 32. A mechanism according to claim 31,wherein the upper component comprises a flange that extends radiallyoutwardly from the upper end of the inner side wall of the uppercomponent and which flange forms the upper annular surface of thecircumferential groove and wherein the upper edge of the outer side wallof the tubular lower component forms the lower annular surface of thecircumferential groove.
 33. A mechanism according to claim 30 in whichthe upper component comprises the upper annular surface and the lowercomponent comprises the lower annular surface.
 34. A mechanism accordingto claim 30 in which the upper component and the lower component definean axial position of the deformable ring with respect to the dart body.35. An activating mechanism for controlling the operation of a downholetool and which comprises: a hollow main body adapted for mounting in adrill-string and through which fluid to the tool can be routed; anactuating sleeve defining a through-flow passage and slidably mounted inthe main body for movement between positions corresponding to athrough-flow mode and a by-pass mode of the mechanism; biasing meansacting on the sleeve to urge it to its position corresponding to thethrough-flow mode of the mechanism; a seat providing access to saidpassage in the through-flow mode of the mechanism; and a deformableactivator capable of being launched down the drill-string to engage theseat and thereby cause pressure upstream of the seat to increase so thatthe deformable activator moves the sleeve to its position correspondingto the by-pass mode of the mechanism; in which the deformable activatorand the seat are arranged to co-operate with each other, when thedeformable activator engages the seat, in such a way that restrictedflow of fluid through the sleeve is maintained when the mechanism is inits by-pass mode; the deformable activator being a deformable dartcomprising: a dart body having an outer side surface portion with acircumferential groove in the side surface portion; an externaldeformable ring which is inwardly deformable, the external deformablering being mounted in the circumferential groove and protrudingoutwardly therefrom such that the deformable ring is capable of seatingon said seat with at least a portion of the dart body below thecircumferential groove projecting downwardly through the seat; whereinthe dart body comprises at least two components including an uppercomponent and a lower component which are assembled together into aunitary construction, further comprising at least one of the followingfeatures: (i) the circumferential groove has an upper annular surfaceand a lower annular surface wherein the upper component comprises theupper annular surface and the lower component comprises the lowerannular surface; (ii) the upper component and the lower component definean axial position of the deformable ring with respect to the dart body.36. An activating mechanism for controlling the operation of a downholetool and which comprises: a hollow main body adapted for mounting in adrill-string and through which fluid to the tool can be routed; anactuating sleeve defining a through-flow passage and slidably mounted inthe main body for movement between positions corresponding to athrough-flow mode and a by-pass mode of the mechanism; biasing meansacting on the sleeve to urge it to its position corresponding to thethrough-flow mode of the mechanism; a seat providing access to saidpassage in the through-flow mode of the mechanism; and a deformableactivator capable of being launched down the drill-string to engage theseat and thereby cause pressure upstream of the seat to increase so thatthe deformable activator moves the sleeve to its position correspondingto the bypass mode of the mechanism: in which the deformable activatorand the seat are arranged to co-operate with each other, when thedeformable activator engages the seat, in such a way that restrictedflow of fluid through the sleeve is maintained when the mechanism is inits bypass mode; the deformable activator being a deformable dartcomprising: a dart body having a outer side surface portion with acircumferential groove in the side surface portion; an externaldeformable ring which is inwardly deformable, the external deformablering being mounted in the circumferential groove and protrudingoutwardly therefrom such that the deformable ring is capable of seatingon said seat with at least a portion of the dart body below thecircumferential groove projecting downwardly through seat, wherein theexternal deformable ring is capable of deforming under load, therebyallowing the entire deformable activator to pass downwardly through theseat; wherein the deformable ring has an outer surface, at least aportion of which tapers downward and radially inwardly.
 37. A mechanismaccording to claim 36, in which the hollow body has at least one by-passport to direct fluid flow laterally of the sleeve and the body when themechanism is in its by-pass mode, and with the sleeve being moved by thedeformable actuator so as to allow access to the by-pass port.
 38. Amechanism according to claim 36, in which a non-deformable activator isprovided, which is capable of being launched down the drill string andto move to a blocking position which blocks by-pass flow of fluid, andthereby causes increase in pressure upstream of the seat.
 39. Amechanism according to claim 36, in which the deformable activator ishollow and is provided with an internal flow control device.
 40. Amechanism according to claim 36, in which the dart body comprises atleast two components including an upper component and a lower componentwhich are assembled together into a unitary construction.
 41. Amechanism according to claim 40 in which the circumferential groove hasan upper annular surface and a lower annular surface and in which theupper component comprises the upper annular surface and the lowercomponent comprises the lower annular surface.
 42. A mechanism accordingto claim 40 in which the upper component and the lower component definean axial position of the deformable ring with respect to the dart body.